This invention relates to solid state electronic imaging devices and, more particularly, to active pixel image sensing devices having improved image quality and fabricated using a CMOS process.
Image sensing devices which operate by performing the steps of sensing incident optical radiation, converting the radiation into charge carriers, and storing the charge carriers in photosensitive material are known in the art. Most conventional image sensing devices are based on charge-coupled device (CCD) technology. Fabricating a CCD-based imager requires a number of specialized manufacturing operations as compared to fabricating other commonly used semiconductor components. Moreover, a CCD-based imager utilizes nonstandard voltages with respect to voltages used by other commonly used semiconductor components necessitating a complex power supply and associated distribution circuitry. These requirements increase the difficulty of integrating circuits employing CCDs into an electronic system and increase the complexity of the fabrication process.
Certain design and fabrication difficulties associated with CCD-based image sensing devices can be overcome with an imager having pixels made from CMOS transistors and fabricated using a CMOS process. However, voltage threshold variations among CMOS devices, on the same substrate, make it difficult to achieve uniform and precise resetting of pixels, and produce fixed-pattern noise in the output signal. A CMOS based image sensing device in which fixed pattern noise attributable to pixel-to-pixel voltage threshold variation is substantially eliminated is desirable. A reduction in fixed pattern noise is directly related to an improved signal to noise ratio (Signal/Noise).
Some known CMOS image sensing devices operate in a current mode and incorporate current mediated pixels, reset the pixels with a current reference and generate a current output. Correlated double sampling, a technique which measures both a reference value and a signal value, can be used to reduce fixed pattern noise. Unfortunately, the reduction of fixed pattern noise is limited due to the non-linear quadratic current mode transfer function of current mode pixels. Other known CMOS image sensing devices use an active pixel operating in a voltage mode including a photo diode, a reset switch, an active device, a row select transistor, and an output select transistor. The voltage mode pixel is reset with a reference voltage and generates a voltage signal as an output. These known devices reset the voltage on the active pixels to the Vdd supply voltage which causes the pixels to exhibit a smeared image when imaging a moving object due to a partial signal which lags on a pixel from a previous frame. This effect is referred to as lag. Known devices are also subject to blooming which is an image quality artifact caused by excess charge from high illumination on one pixel spilling over onto neighboring pixels. Some prior art devices use a source follower voltage output but can not effectively eliminate fixed pattern noise. Other known devices use voltage signals on the output bus but require more power to operate at an equivalent speed performance level compared to the present invention. An additional disadvantage to using voltage signals on the output bus is the requirement for more precise capacitors in the output amplifier circuitry. More precise capacitors require fabrication process enhancements typically adding additional manufacturing steps. There is a need in the art for CMOS imaging devices with improved image quality including improved signal to noise ratio.
It is an object of the present invention to provide a solid-state imaging device including a large array of pixels which can be fabricated using a CMOS process and which yields a signal with lower levels of fixed-pattern noise caused by pixel-to-pixel variation in operating parameters including voltage variations on the column and output busses.
It is a further object of the present invention to provide a highly linear voltage to current converter which allows the use of correlated double sampling to minimize fixed pattern noise.
It is another object of the present invention to improve image quality by controlling the pixel reset process in order to reduce image smearing caused by lag of a partial on a pixel between fields and to reduce pixel blooming artifacts due to the generation of excess photo carriers in the presence of high levels of illumination.
In accordance with the present invention, the foregoing objectives are achieved by an image sensing device including one or more pixels with CMOS components in which a pixel bias reference voltage is utilized to reset the pixels instead of the Vdd supply voltage and a linear voltage to current converting circuit with bus voltage independent biasing. The image sensing device further includes active pixels operated in a voltage mode to produce a pixel output voltage which contributes to the output signal using a source follower amplifier; a level shifter to shift the output voltage level to a higher voltage, a coupling device which is periodically clamped to generate a reference current and to couple the higher voltage to a buffer device. The output of the buffer drives a linear mode current controlling device which is biased to linearly convert the voltage input into a current which is stored in a current mirror. With such an arrangement, the conversion of a massive array of pixels occurs in a manner which reduces additional noise and signal variation due to IR drops along the row and column output busses. Additionally, the image sensing device includes a reference current mirror which stores a reference current which is proportional to a predetermined clamp voltage and a signal current mirror which stores a signal current which is indicative of the output voltage signal from the active pixel. Both the reference current mirror and the signal current mirror supply current inputs to a differential amplifier to remove most of the fixed pattern noise from the output signal.
In accordance with the present invention, the foregoing objectives are achieved by a method of converting an output voltage from an active pixel in an image sensing device into a current including the steps of: providing a bias voltage for biasing a linear mode current controlling device into a linear mode; periodically clamping an input to a buffer device to a predetermined clamp voltage; shifting the output voltage level to a higher voltage; coupling the higher voltage to the buffer device; generating an output signal from the buffer device for controlling the linear mode current controlling device; and generating a current signal flowing through the linear mode current controlling device which is indicative of the output voltage from the active pixel.